Anti-HPA-1a Fetal-Neonatal AlloImmune Thrombocytopenia: Reframing Diagnostics, Pathophysiology, and Management

Maternal alloantibodies directed to HPA-1a on fetal platelets can induce fetal-neonatal alloimmune thrombocytopenia (FNAIT) which causes intracranial hemorrhage in 10-20% of fetuses/newborns. Presentation is usually unexpected and identified by neonatal bleeding, with implications for future pregnancies. This review synthesizes advances in diagnosis, pathophysiology, and management that reshape understanding of anti-HPA-1a-mediated FNAIT. Genomic and serologic testing, together with cell-free fetal DNA for fetal HPA typing, allow accurate identification of at-risk pregnancies. Among HPA-1bb women, those who carry DRB3*01:01 are at greatest risk of forming clinically-significant anti-HPA-1a. Not only anti-HPA-1a levels but also structural features, particularly decreased Fc-fucosylation enhancing FcγR-mediated effector functions, more accurately determine disease severity. Furthermore, increased Fc-galactosylation may contribute by enhancing complement activation. Fab-mediated effects impact platelets, megakaryocytes, trophoblasts, and endothelial cells. Taken together, this explains why anti-HPA-1a levels and neonatal platelet counts alone do not reliably predict bleeding including intracranial hemorrhage. Anti-HPA-1a also induces placental inflammation increasing risks of fetal growth restriction and long-term neurodevelopmental impairment, e.g. autism. Neonatal management involves random donor and matched platelet transfusions, and also IVIG if needed. Antenatal IVIG, with/without prednisone administered in an affected pregnancy typically increases fetal platelet counts with management strategies varying internationally. Blocking FcRn has emerged as an alternative approach to both reduce maternal anti-HPAa-1a levels and inhibit its transplacental transfer. Whether antenatal treatment reduces placental inflammation requires further study. These developments support the importance of identifying predictive biomarkers of fetal risk to guide antenatal management and of preventing affected pregnancies ideally by screening all pregnancies followed by prophylaxis.